268 PROFESSOR WILLIAM THOMSON ON THE 
engine and produce mechanical effect, there will be less heat emitted from the 
parts of the circuit not affected by the source than is taken in from the source, by 
an amount precisely equivalent to the mechanical effect produced; since JouLE 
demonstrates experimentally that a current from any kind of source, driving an 
engine, produces in the engine just as much less heat than it would produce in a 
fixed wire exercising the same resistance as is equivalent to the mechanical effect 
produced by the engine. 
18. The equality of thermal effects, resulting from equal causes through very 
different means, is beautifully illustrated by the following statement, drawn from 
Mr JouLr’s paper on magneto-electricity. 
Let there be three equal and similar galvanic batteries, furnished with equal 
and similar electrodes: let A, and B, be the terminations of the electrodes (or wires 
connected with the two poles) of the first battery; A, and B, the terminations 
of the corresponding electrodes of the second ; and A, and B, of the third battery. 
Let A, and B, be connected with the extremities of a long fixed wire; let A, and 
B, be connected with the “ poles” of an electrolytic apparatus for the decompo- 
sition of water; and let A, and B, be connected with the poles (or ports as they 
might be called) of an electro-magnetic engine. Then if the length of the wire 
between A, and B,, and the speed of the engine between A, and B,, be so adjusted 
that the strength of the current (which, for simplicity, we may suppose to be con- 
tinuous and perfectly uniform in each case) may be the same in the three circuits, 
there will be more heat given out in any time in the wire between A, and B, than 
in the electrolytic apparatus between A, and B,, or the working engine between 
A,and B,. But if the hydrogen were allowed to burn in the oxygen, within the 
electrolytic vessel, and the engine to waste all its work without producing any 
other than thermal effects (as it would do, for instance, if all its work were spent 
in continuously agitating a limited fluid mass), the total heat emitted would be 
precisely the same in each of these two pieces of apparatus as in the wire between 
received from Mr Jourz on the 8th of July 1847. “In Prnrier’s experiment on cold produced at 
the bismuth and antimony solder, we have an instance of the conversion of heat into the mechanical 
force of the current,’ which must have been meant as an answer to a remark I had made, that no 
evidence could be adduced to shew that heat is ever put out of existence. I now fully admit the 
force of that answer, but it would require a proof that there is more heat put out of existence at the 
heated soldering than is ereated at the cold soldering, to make the “evidence” be experimental. 
That this is the case I think is certain, because the statements of § 16 in the text are demonstrated 
consequences of the first fundamental proposition; but it is still to be remarked, that neither in this 
nor in any other case of the production of mechanical effect from purely thermal agency, has the 
ceasing to exist of an equivalent quantity of heat been demonstrated otherwise than theoretically. 
It would be a very great step in the experimental illustration (or verification, for those who consider 
such to be necessary) of the dynamical theory of heat, to actually shew, im any one case, a loss of 
heat ; and it might be done by operating through a very considerable range of temperatures with a 
good air-engine or steam-engine, not allowed to waste its work in friction. As will be seen in 
Part II. of this paper, no experiment of any kind could shew a considerable loss of heat without 
employing bodies differing considerably in temperature ; for instance, a loss of as much as -098, or 
about one-tenth of the whole heat used, if the temperature of all the bodies used be between 0° and 
30° cent, 

